The present invention relates to improved processes for drying polybenzazole fibers. Polybenzazole ("PBZ") fibers include polybenzoxazole ("PBO") or polybenzothiazole ("PBT") fibers.
Lyotropic liquid crystalline PBZ is typically made into fibers by dry-jet, wet-spinning techniques, in which a solution that contains the PBZ polymer and an acid solvent (known as a "dope") is spun through a spinneret to form dope filaments, that are combined into one or more dope fibers. These dope fibers are drawn across an air gap, and then contacted with a fluid that dilutes the solvent and is a non-solvent for the polymer. This contact with fluid causes the polymer to separate from the solvent. See jointly owned, Allowed, U.S. Pat. No. 5,296,185 (Method for Spinning a Polybenzazole Fiber) and U.S. Pat. No. 5,294,390 (Method for Rapid Spinning of a Polybenzazole Fiber), which are incorporated by reference, for a description of the PBZ fiber spinning process.
The process of separating the PBZ polymer in the dope fiber from the solvent in the dope fiber is known as coagulation. After coagulation, most of the remaining residual solvent is washed/leached from the coagulated fiber, leaving the fiber wet. See jointly owned, U.S. patent application Ser. No. 08/110,149 (Improved Process for Coagulation and Washing of Polybenzazole Fibers), which is incorporated by reference, for a description of the coagulation process.
Polybenzazole fibers typically contain a very high degree of residual moisture after they are washed. The residual moisture content is frequently between 30 and 200 weight percent, and may even be higher in some fibers. The percent residual moisture content, (hereinafter percent RMC) is calculated on a parts per hundred basis as follows: EQU [(initial fiber weight-dried fiber weight)/dried fiber weight].times.100%.
For many reasons it is desirable to reduce the amount of residual moisture in the fiber by drying the fiber. One of the reasons it is desirable to reduce the amount of residual moisture in the fiber is to enable the fiber to be heat treated without damaging the fiber. Heat treating of dried fibers can be and is done to improve the fibers' physical properties. It has been discovered that PBZ fiber can be damaged by exposing it to the typical amount of heat (about 400.degree. C.) used in heat treating while the fiber contains more than about twelve percent RMC. Therefore, in order to be heat treated without being damaged, a PBZ fiber usually must have a percent RMC of less than about twelve percent.
In order to reduce the amount of residual moisture in the fiber to below twelve percent RMC prior to the fiber being heat treated, it has heretofore been necessary to dry the fiber for over 40 hours at 65.degree. C. It is economically undesirable to dry the fiber at this low temperature, because low temperature drying is, as noted above, very time-consuming and thus, very costly. It has been found that increasing the temperature of the drying process will speed up the drying process but can also cause damage to the fiber. This heat induced damage appears as visible voids in the fiber. These voids are highly undesirable for all PBZ fibers. Therefore, it is desirable to develop a process that allows for rapid drying of PBZ fiber without causing damage to the fibers.